How common are Polymyositis and dermatomyositis
What are some of the epidemiologic features of Polymyositis and dermatomyositis?
• Annual incidence of 2–19 cases/million.
• Peak age of onset is bimodal in distribution for DM: one peak at 5 to 15 years of age, and the other at 45 to 65 years. PM rarely occurs in patients aged <15 years, with mean age of onset 50 to 60 years.
• The female to male ratio is 2 to 3:1 overall. The female to male ratio in JDM is close to 1:1. In overlap myositis associated with other connective tissue disorders it is 8 to 10:1.
• In the United States, African-Americans are affected more commonly than Caucasians at a ratio of 3 to 4:1 in some studies.
